534 CHEMISTRY [IRON GROUP. Chlorides. Cr 8 Cl 6 Mn,Cl 8 Fe 2 Cl 6 CO.CL CrF 6 ? MnF v/r*^i^ - a o Mn 2 Cl 4 Mn ? Cl 6 MnCl 4 Fe 2 Cl 4 Co 2 Cl 4 Ni 2 Cl 4 Their chlorides, and, in fact, their halogen compounds generally, are all easily soluble in water, with the exception of sublimed chromic chloride, Cr 2 Cl 6 , which is insoluble in cold, and is only slowly dissolved by boiling water; but if the cold solution contains a minute quantity of chromous chloride, Cr 2 Cl 4 , the chromic chloride dissolves immediately with development of heat, forming a green solution identical with that produced by dissolving chromic hydroxide in hydrochloric acid; this effect is, perhaps, due to the formation in the first instance of an intermediate chloride, or combination of the two chlorides, which is decomposable by water. The chlorides which correspond in composi tion to ferrous chloride, Fe 2 Cl 4> are all perfectly stable, and volatilize without decomposition ; chromic and ferric chlorides are also highly stable bodies, but the remaining chlorides are exceedingly unstable. Chromium hexafl uoride, CrF 6 , is a very volatile blood-red liquid which is decom posed by water. Their oxides, with the exception of chromium trioxide or chromic anhydride, Cr0 3 , and hydroxides are all insoluble in water, but readily dissolve in acids. Chromic oxide, Cr 2 O 3 , is green, and chromium trioxide has a magnificent scarlet colour. Ferric oxide, Fe 2 3 , has a reddish-brown colour ; the oxides of manganese are brownish black ; and the oxides of nickel and cobalt are all intensely black like cupric oxide. Chromic and ferric oxides form crystals isomorphous with those of aluniinic oxide, A1 2 3 . The affinity of chromium to oxygen is so great that chromous oxide, Cr 2 2 , is apparently incapable of existing, and it is difficult to obtain ferrous oxide, Fe 2 2 , on account of the readiness with which it is oxidized to ferric oxide, Fe 2 O 3 . Ferric oxide, like chromic oxide, Cr 2 O 3 , is a body of great stability, but unlike the latter it is converted into a lowsr oxide, ferroso-ferric oxide, Fe 3 O 4 , on ignition. The most stable oxide of manganese is the corresponding oxide, Mn 3 O 4 ; but manganous oxide, Mn 2 2 , is less readily oxidized than ferrous oxide, and manganic oxide is less stable than ferric oxide ; manganese is the only metal of the iron group which forms a dioxide, or, as it is usually termed, a peroxide. Cobaltous oxide, Co 2 O 2 , absorbs oxygen when heated to dull redness in air, and is converted into the oxide Co 3 4 , but when more strongly heated this oxide is reconverted into cobaltous oxide, which is also obtained on igniting cobaltic oxide, Co 2 3 . Nickelous oxide, Ni 2 2 , exhibits no tendency to absorb oxygen, and the higher oxide is very readily decomposed by heat. From this it will be evident that the affinity to oxygen diminishes rapidly from chromium to nickel. Similar differences are noticeable between their hydroxides. Thus, the tendency of chromous hydroxide, Cr 2 (OH) 4 , to become converted into chromic hydroxide, Cr^OH)^ is so great that it decomposes water even at ordinary tempera tures with evolution of hydrogen: Cr 2 (OH) 4 + 20H 2 = Cr 2 (OH) 6 + H 2 . Ferrous hydroxide does not decompose water in this manner, but in presence of both air and water it is quickly converted into ferric hydroxide : 2Fe 2 (OH) 4 + 4OH 2 + 2 = 2Fe 2 (OH) (5 + 20H 2 ; manganous hydroxide behaves similarly, but oxidizes much less rapidly ; cobaltous hydroxide manifests but little tendency to form the higher hydroxide, and nickelous hydroxide is unaffected, when placed in contact with air and water. The two last named hydroxides, however, may be converted into the higher hydroxides by treatment with a solution of sodium hypochlorite. Their hydroxides are without difficulty converted into the corresponding oxides, giving off water when very gently heated ; the behaviour of chromous hydroxide is peculiar, as, when ignited, it is converted into chromic oxide with evolution of hydrogen : Cr ? (OH) 4 = Cr 2 O 3 + OH 2 + H 2 . Many hydroxides are known intermediate in composition between the oxides and the highest hydroxides, such, for example, as are represented by the formulae Fe 2 0(OH) 4 and Fe 2 O 2 (OH) 2 , which obviously are intermediate in composition between ferric oxide, Fe 2 3 , and ferric hydrox ide, Fe. 2 (OH) . With the exception of chromium trioxide, the oxides and hydroxides of the metals now under consideration all exhibit marked basic properties ; but chromic hydroxide, like aluminie hydroxide, also has feeble acid properties, dissolving in solutions of the alkalies ; and cobaltous and nickelous hydroxides, like cupric hydroxide, readily dissolve in ammonia, the solution of the former being brownish-red, and that of the latter blue. The oxides and hydroxides which correspond in composition with ferrous oxide and hydroxide furnish stable salts such as ferrous sulphate, Fe 2 (SO 4 ) 2 . The chromous salts, however, are but little known; they form either red or blue solutions, which absorb oxygen with avidity, and dissolve nitric oxide gas, forming dark brown solutions. The ferrous salts form pale green solutions, which slowly absorb oxygen, and readily dissolve nitric oxide (p. 513). The manganous salts are of a pale pink colour, and their solutions are scarcely altered in con tact with oxygen. Solutions of cobaltous salts have a rose- red colour, unless they are very concentrated or mixed with a strong acid, in which case they are blue. Nickelous salts are green. The solutions of cobaltous and uickelous salts are permanent in air. A second series of chromium and iron salts correspond ing to the oxides Cr 2 O 3 and Fe 2 3 are readily obtained, as they are of great stability. Solutions of chromic salts are green or violet ; solutions of ferric salts are yellow. Chromic salts such as chromic sulphate, Cr 2 (S0 4 ) 3 , for example, are reduced to chromous salts by the action of the nascent hydrogen produced when zinc is immersed in their solutions, but they cannot be thus reduced by the aid of iron ; ferric salts, however, are readily reduced to ferrous salts even by feeble reducing agents such as hydrogen sulphide. Man ganic and cobaltic salts corresponding to the chromic and ferric salts may be obtained, but they are extremely un stable, and nickelic salts appear to be incapable of existing, so that, on treating the higher oxides and hydroxides of manganese, &c., with acids, salts derived from manganous oxide and its congeners are usually obtained. Thus, nickelic oxide, Ni 2 O 3 , is converted by the action of sulphuric acid into nickelous sulphate, with evolution of oxygen ; similarly, when manganic oxide, Mn 2 3 , manganic peroxide, MnO 2 , or cobaltic oxide, Co 2 O 3 , is carefully dissolved in hydro chloric acid, solutions are obtained which apparently contain the corresponding chlorides, Mn 2 Cl 6 , MnCl 4 , and Co 2 Cl 6 , but these rapidly decompose with evolution of chlorine, leaving manganous chloride, Mn 2 Cl 4 , or cobaltous chloride Co.,Cl 4 . The oxides which correspond in composition to ferroso-ferric oxide, Fe 3 O 4 , behave with acids as if mixtures of the two oxides. Salts corresponding to chromium tri oxide are not obtainable, for on treatment with sulphuric acid it is converted into chromic sulphate, with evolution of oxygen ; and by the action of hydrochloric acid it is con verted into chromic chloride, with evolution of chlorine. Like all feebly positive metals, the metals of the iron group form numerous basic salts, iron and chromium being espe cially characterized by the formation of basic chromic and ferric salts ; a remarkable series of ferric and chromic " mixed salts," such, for example, as ferric tetraceto-dini-
trate, Fe 2 (NO 3 ) 2 (C 2 H 3 2 ) 4 , and chromic pentacetonitrate,